1. Field of the Invention
The present invention relates to a driving method and a driving circuit for a piezoelectric transformer, a cold cathode tube emission device using a cold cathode tube as a load for a piezoelectric transformer, a liquid crystal panel whose luminance is controlled by the cold cathode tube emission device incorporated therein and an apparatus provided with the liquid crystal panel such as a mobile telephone, a communication terminal etc.
2. Description of the Related Art
In the following, a driving circuit using a conventional piezoelectric transformer will be explained.
Generally in a piezoelectric transformer, due to the impedance of a load connected to the secondary side, a voltage step-up ratio indicating a voltage to be output to the secondary side relative to a voltage entering the primary side is changed, and the driving efficiency, shown by the electric power sent to the secondary side relative to the electric power entering the primary side, also is changed, so that a driving frequency for obtaining a maximum voltage step-up ratio and maximum driving efficiency also is changed. In other words, in order to drive a piezoelectric transformer efficiently at a predetermined voltage step-up ratio, the driving frequency must be set according to the impedance of the load to be connected.
For example, in the case of using a cold cathode tube as a load for a piezoelectric transformer, since a cold cathode tube generally shows a high impedance of not less than several hundreds of M xcexa9 until it starts to light and an abrupt impedance decrease from several hundreds of xcexa9 to several tens of xcexa9 after lighting, in order to light the cold cathode tube efficiently by using the piezoelectric transformer, the frequency and the voltage level of an AC voltage applied to the primary side of the piezoelectric transformer should be changed before and after lighting.
As a conventional technique to achieve this purpose, cold cathode tube driving devices disclosed in JP 6(1994)-167694A and others are known, and FIG. 6 shows a block diagram of a driving device disclosed in this publication.
In FIG. 6, an output signal from a free-running multivibrator 106 is amplified by a current amplification circuit 107, and the output signal whose voltage is stepped up further by a wire-wound transformer 108, if necessary, is applied to the primary side of a piezoelectric transformer 101. A cold cathode tube 102 is connected as a load to the secondary side output of the piezoelectric transformer 101, and an electric current flowing in the cold cathode tube 102 is detected by a load current detection circuit 109. The detected current level is converted into a voltage and is input to one of the input terminals of an integration circuit 104 via an AC voltage rectification circuit 110. To the other input terminal, a signal from a variable voltage device 103 is supplied, so that an oscillating frequency of the free-running multivibrator 106 is controlled from the integration circuit 104 via a voltage level shift circuit 105.
In order to light the cold cathode tube 102 serving as the load of the piezoelectric transformer 101, the voltage applied to the piezoelectric transformer 101 is set by the variable voltage device 103 and the voltage level shift circuit 105 etc., and a driving frequency of the piezoelectric transformer 101 is swept so as to light the cold cathode tube 102. After lighting, the driving frequency of the piezoelectric transformer 101 is swept further, and furthermore, according to the current level detected by the load current detection circuit 109 etc., the voltage applied to the piezoelectric transformer 101 is controlled by the variable voltage device 103 and the voltage level shift circuit 105 etc., so that the emission luminance of the cold cathode tube 102 is adjusted.
When a load with a variable impedance such as a cold cathode tube is connected to a piezoelectric transformer, according to a conventional driving method, before the cold cathode tube starts to light, an AC voltage having a large amplitude corresponding to the load in a high impedance state is applied to the secondary side of the piezoelectric transformer at a frequency that is higher than a resonance frequency in an open state. The impedance of the cold cathode tube is reduced according to a change in the lighting state, and the flowing electric current is increased. The cold cathode tube can be lit steadily by detecting this electric current flowing in the cold cathode tube and sweeping the frequency to its low frequency side, and also by changing the amplitude of the applied voltage to become smaller. As a result, there is a problem that the piezoelectric transformer must be driven in the state of low driving efficiency.
Furthermore, corresponding to the high impedance of the cold cathode tube before it starts to light, a high voltage is applied to the piezoelectric transformer. However, corresponding to the reduction of the impedance due to the lighting of the cold cathode tube, a control operation to reduce the applied voltage is performed, so that there is a possibility of momentarily applying a high voltage to the cold cathode tube in a low impedance state. At this time, a strong distortion occurs in the piezoelectric transformer due to a large electric current flowing in the piezoelectric transformer. In particular, when the electric power per volume of the piezoelectric transformer is large, the distortion effected on the piezoelectric transformer may destroy the piezoelectric transformer itself or cause mechanical damage leading to the destruction.
Furthermore, a piezoelectric transformer has a characteristic variance arising from the shape or the material property etc. with respect to frequency characteristics of impedance or an admittance seen from the primary side, or a resonance frequency etc. FIG. 7 is a graph showing the relationship of a voltage step-up ratio and driving efficiency relative to a driving frequency of a piezoelectric transformer. In FIG. 7, the horizontal axis shows driving frequency of the piezoelectric transformer, and the vertical axis on the left side shows a voltage step-up ratio indicating a ratio of a voltage output from the secondary side relative to the voltage applied to the primary side of the piezoelectric transformer, and furthermore, the vertical axis on the right side shows driving efficiency, indicating a ratio of electric power output from the secondary side relative to the electric power applied to the primary side of the piezoelectric transformer. FIG. 7 shows that the voltage step-up ratio and the frequency efficiency of the piezoelectric transformer are variable within a certain form tolerance.
In the case where frequency characteristics of the voltage step-up ratio or the driving efficiency relative to the driving frequency are different as shown in FIG. 7, for example, a piezoelectric transformer achieving a voltage step-up ratio of a maximum value xcex3max with a driving frequency fxcex32 can be driven with maximum driving efficiency xcex7max by driving at a frequency fxcex72. However, when it is driven at a frequency fxcex71 or a frequency fxcex73, the driving efficiency becomes lower than xcex7max.
In this way, due to the characteristic variance arising from the shape or the material property etc. of the piezoelectric transformer, there is a problem that the piezoelectric transformer cannot be driven with the maximum driving efficiency only by using the driving frequency or the driving voltage predetermined by the driving circuit.
Furthermore, when a liquid crystal panel with a built-in cold cathode tube driving device is incorporated into an apparatus such as a mobile telephone or communication equipment etc., a sweep of the driving frequency becomes a problem. In other words, when a sweep of the driving frequency is performed before starting to light over to the lighting state not continuously but switching the frequency discretely, higher harmonics arise at the switching point of the frequency, which leads to noise that affects the operation of the apparatus. Furthermore, due to the sweep of the driving frequency, a cross modulation may be generated in the carrier frequency of the communication equipment, so that there is fear that the communication cannot be carried out normally.
The present invention is conceived in light of the aforementioned problems, and its object is to provide a driving method and a driving circuit for a piezoelectric transformer capable of driving the piezoelectric transformer with high driving efficiency independently of an impedance change of a load such as a cold cathode tube by corresponding to the characteristic variance of the piezoelectric transformer, thereby also suppressing mechanical damage that might be imposed on the piezoelectric transformer itself even if the load changes abruptly.
It is another object of the present invention to provide a cold cathode tube emission device that controls the emission of a cold cathode tube by the driving circuit of the piezoelectric transformer, a liquid crystal panel provided with this cold cathode tube emission device, and an apparatus provided with this liquid crystal panel such as a mobile telephone, a communication terminal etc.
To achieve the above object, a first method for driving a piezoelectric transformer according to the present invention is a method for driving a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode. The method is characterized by the steps of driving the piezoelectric transformer with a predetermined first frequency, starting a power supply for a load connected to the secondary side of the piezoelectric transformer, and when an impedance of the load changes and reaches a predetermined impedance during the power supply, changing the driving frequency of the piezoelectric transformer from the first frequency without a sweep to a predetermined second frequency and driving the piezoelectric transformer.
To achieve the above object, a second method for driving a piezoelectric transformer according to the present invention is a method for driving a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode. The method is characterized by the steps of, before supplying a first electric power steadily to a load connected to the piezoelectric transformer, supplying the load with a second electric power that is sufficiently smaller than the first electric power to detect characteristics of the piezoelectric transformer, and setting a driving frequency of the piezoelectric transformer for the time when the first electric power is supplied steadily.
In the second driving method, the load changes from a high impedance state to a low impedance state according to an increase in the amount of electricity to be supplied, and the first electric power has a level needed to change the load to be in a low impedance state, and the second electric power has only a level to keep the load in a high impedance state.
Furthermore, in the second driving method, it is preferable to drive the piezoelectric transformer with a first frequency when starting the power supply for the load, and to drive the piezoelectric transformer by changing the driving frequency without a frequency sweep to a second frequency when supplying the load steadily with electric power.
To achieve the above object, a third method for driving a piezoelectric transformer according to the present invention is a method for driving a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode. The method is characterized by the steps of classifying the piezoelectric transformer in advance by a certain characteristic parameter, determining a second frequency by assuming characteristics of the piezoelectric transformer based on the result of driving the piezoelectric transformer with a predetermined first frequency and on the result of classification by the characteristic parameter, starting a power supply for a load connected to the secondary side of the piezoelectric transformer, and when an impedance of the load changes and reaches a predetermined impedance during the power supply, changing the driving frequency of the piezoelectric transformer from the first frequency without a sweep to the second frequency and driving the piezoelectric transformer.
In the third driving method, it is preferable to classify the piezoelectric transformer by the certain characteristic parameter and also to classify the load by the same characteristic parameter or another characteristic parameter, and to determine the second frequency based on the result of classifying the piezoelectric transformer and the result of classifying the load.
In the first, second and third driving methods, a cold cathode tube is used as the load.
Furthermore, in the first, second and third driving methods, it is preferable to perform the driving control of the piezoelectric transformer by using a microcomputer and its peripheral equipment.
To achieve the above object, a first driving circuit of a piezoelectric transformer according to the present invention includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a load output detection part for detecting that an impedance of a load connected to the secondary side of the piezoelectric transformer reaches a predetermined value, and a control part for setting a driving frequency of the piezoelectric transformer as a first frequency when starting a power supply for the load and changing the driving frequency from the first frequency to a second frequency without a sweep according to the result of detection by the load output detection part.
To achieve the above object, a second driving circuit of a piezoelectric transformer according to the present invention includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a load output detection part for detecting that an impedance of a load connected to the secondary side of the piezoelectric transformer reaches a predetermined value, a transformer characteristic detection part for detecting characteristics of the piezoelectric transformer, and a control part for performing a driving control of the piezoelectric transformer with a second electric power that is sufficiently smaller than a first electric power before supplying the load steadily with the first electric power, and based on the characteristics of the piezoelectric transformer detected by the transformer characteristic detection part, performing a setting control of a driving frequency of the piezoelectric transformer according to the result of detection by the load output detection part when the first electric power is supplied steadily.
In the second driving circuit, the load changes from a high impedance state to a low impedance state according to an increase in the amount of electricity to be supplied, and the first electric power has a level needed to change the load to be in a low impedance state, and the second electric power has only a level to keep the load in a high impedance state.
Furthermore, in the second driving circuit, it is preferable that the control part performs the driving control of the piezoelectric transformer with a first frequency when starting a power supply for the load and performs the driving control of the piezoelectric transformer by changing the driving frequency without a sweep to a second frequency according to the result of detection by the load output detection part when supplying the load steadily with electric power.
To achieve the above object, a third driving circuit of a piezoelectric transformer according to the present invention includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a load output detection part for detecting that an impedance of a load connected to the secondary side of the piezoelectric transformer reaches a predetermined value, a transformer characteristic detection part for detecting characteristics of the piezoelectric transformer, and a control part for determining a second frequency by assuming characteristics of the piezoelectric transformer classified in advance by a certain characteristic parameter, based on the result of classification by the characteristic parameter and the result of detection by the transformer characteristic detection part at the time when the piezoelectric transformer is driven with a predetermined first frequency, and setting a driving frequency of the piezoelectric transformer as the first frequency when starting a power supply for the load and changing the driving frequency from the first frequency to the second frequency without a sweep according to the result of detection by the load output detection part.
In the third driving circuit, it is preferable that the load is classified by the characteristic parameter or another characteristic parameter, and that the control part determines the second frequency based on the results of classifying the piezoelectric transformer and the load.
In the first, second, and third driving circuits, a cold cathode tube is used as the load.
Furthermore, in the first, second and third driving circuits, it is preferable that the control part includes a microcomputer and its peripheral equipment.
To achieve the above object, a first cold cathode tube emission device according to the present invention includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a cold cathode tube connected to the secondary side of the piezoelectric transformer, a cold cathode tube output detection part for detecting that an impedance of the cold cathode tube reaches a predetermined value, and a control part for setting a driving frequency of the piezoelectric transformer as a first frequency when starting a power supply for the cold cathode tube and changing the driving frequency from the first frequency to the second frequency without a sweep according to the result of detection by the cold cathode tube output detection part.
To achieve the above object, a second cold cathode tube emission device according to the present invention includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a cold cathode tube connected to the secondary side of the piezoelectric transformer, a cold cathode tube output detection part for detecting that an impedance of the cold cathode tube reaches a predetermined value, a transformer characteristic detection part for detecting characteristics of the piezoelectric transformer, and a control part for performing a driving control of the piezoelectric transformer with a second electric power having a level of not lighting the cold cathode tube before supplying the cold cathode tube steadily with a first electric power, and based on the characteristics of the piezoelectric transformer detected by the transformer characteristic detection part, performing a setting control of a driving frequency of the piezoelectric transformer according to the result of detection by the cold cathode tube output detection part when the first electric power is supplied steadily.
To achieve the above object, a third cold cathode tube emission device according to the present invention includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a cold cathode tube connected to the secondary side of the piezoelectric transformer, a cold cathode tube output detection part for detecting that an impedance of the cold cathode tube reaches a predetermined value, a transformer characteristic detection part for detecting characteristics of the piezoelectric transformer, and a control part for determining a second frequency by assuming characteristics of the piezoelectric transformer classified in advance by a certain characteristic parameter, based on the result of classification by the characteristic parameter and the result of detection by the transformer characteristic detection part when driving the piezoelectric transformer with a predetermined first frequency, and setting a driving frequency of the piezoelectric transformer as the first frequency when starting a power supply for the cold cathode tube and changing the driving frequency from the first frequency to a second frequency without a sweep according to the result of detection by the cold cathode tube output detection part.
To achieve the above object, a first liquid crystal panel according to the present invention is a liquid crystal panel whose luminance is controlled by a built-in cold cathode tube emission device, and the cold cathode tube emission device is characterized by including at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a cold cathode tube connected to the secondary side of the piezoelectric transformer, a cold cathode tube output detection part for detecting that an impedance of the cold cathode tube reaches a predetermined value, and a control part for setting a driving frequency of the piezoelectric transformer as a first frequency when starting a power supply for the cold cathode tube and changing the driving frequency from the first frequency to a second frequency without a sweep according to the result of detection by the cold cathode tube output detection part.
To achieve the above object, a second liquid crystal panel according to the present invention is a liquid crystal panel whose luminance is controlled by a built-in cold cathode tube emission device, and the cold cathode tube emission device includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a cold cathode tube connected to the secondary side of the piezoelectric transformer, a cold cathode tube output detection part for detecting that an impedance of the cold cathode tube reaches a predetermined value, a transformer characteristic detection part for detecting characteristics of the piezoelectric transformer, and a control part for performing a driving control of the piezoelectric transformer with a second electric power having a level of not lighting the cold cathode tube before supplying the cold cathode tube steadily with a first electric power, and based on the characteristics of the piezoelectric transformer detected by the transformer characteristic detection part, performing a setting control of a driving frequency of the piezoelectric transformer according to the result of detection by the cold cathode tube output detection part when the first electric power is supplied steadily.
To achieve the above object, a third liquid crystal panel according to the present invention is a liquid crystal panel whose luminance is controlled by a built-in cold cathode tube emission device, and the cold cathode tube emission device includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a cold cathode tube connected to the secondary side of the piezoelectric transformer, a cold cathode tube output detection part for detecting that an impedance of the cold cathode tube reaches a predetermined value, a transformer characteristic detection part for detecting characteristics of the piezoelectric transformer, and a control part for determining a second frequency by assuming characteristics of the piezoelectric transformer classified in advance by a certain characteristic parameter, based on the result of classification by the characteristic parameter and the result of detection by the transformer characteristic detection part when driving the piezoelectric transformer with a predetermined first frequency, and setting a driving frequency of the piezoelectric transformer as the first frequency when starting a power supply for the cold cathode tube and changing the driving frequency from the first frequency to the second frequency without a sweep according to the result of detection by the cold cathode tube output detection part.
To achieve the above object, a first liquid crystal panel built-in apparatus according to the present invention is an apparatus incorporated with a liquid crystal panel whose luminance is controlled by a built-in cold cathode tube emission device, and the cold cathode tube emission device includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a cold cathode tube connected to the secondary side of the piezoelectric transformer, a cold cathode tube output detection part for detecting that an impedance of the cold cathode tube reaches a predetermined value, and a control part for setting a driving frequency of the piezoelectric transformer as a first frequency when starting a power supply for the cold cathode tube and changing the driving frequency from the first frequency to a second frequency without a sweep according to the result of detection by the cold cathode tube output detection part.
To achieve the above object, a second liquid crystal panel built-in apparatus according to the present invention is an apparatus incorporated with a liquid crystal panel whose luminance is controlled by a built-in cold cathode tube emission device, and the cold cathode tube emission device includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a cold cathode tube connected to the secondary side of the piezoelectric transformer, a cold cathode tube output detection part for detecting that an impedance of the cold cathode tube reaches a predetermined value, a transformer characteristic detection part for detecting characteristics of the piezoelectric transformer, and a control part for performing a driving control of the piezoelectric transformer with a second electric power having a level of not lighting the cold cathode tube before supplying the cold cathode tube steadily with a first electric power, and based on the characteristics of the piezoelectric transformer detected by the transformer characteristic detection part, performing a setting control of a driving frequency of the piezoelectric transformer according to the result of detection by the cold cathode tube output detection part when the first electric power is supplied steadily.
To achieve the above object, a third liquid crystal panel built-in apparatus according to the present invention is an apparatus incorporated with a liquid crystal panel whose luminance is controlled by a built-in cold cathode tube emission device, and the cold cathode tube emission device includes at least a piezoelectric transformer having a primary side electrode and a secondary side electrode formed on a piezoelectric element, in which an AC voltage entering from the primary side electrode is converted and output from the secondary side electrode, a cold cathode tube connected to the secondary side of the piezoelectric transformer, a cold cathode tube output detection part for detecting that an impedance of the cold cathode tube reaches a predetermined value, a transformer characteristic detection part for detecting characteristics of the piezoelectric transformer, and a control part for determining a second frequency by assuming characteristics of the piezoelectric transformer classified in advance by a certain characteristic parameter, based on the result of classification by the characteristic parameter and the result of detection by the transformer characteristic detection part when driving the piezoelectric transformer with a predetermined first frequency, and setting a driving frequency of the piezoelectric transformer as the first frequency when starting a power supply for the cold cathode tube and changing the driving frequency from the first frequency to the second frequency without a sweep according to the result of detection by the cold cathode tube output detection part.
According to the first driving method and circuit of the piezoelectric transformer, the piezoelectric transformer can be driven with high driving efficiency independently of an impedance change of the load. Furthermore, even if the impedance of the load changes abruptly, a large amount of electric current is less likely to flow to the piezoelectric transformer, so that a strong distortion occurring in the piezoelectric transformer can be suppressed, and mechanical damage imposed on the piezoelectric transformer itself also can be suppressed.
According to the second driving method and circuit of the piezoelectric transformer, before supplying the load steadily with first electric power (for example, several kV order by voltage), second electric power (for example, several V order by voltage) that is sufficiently smaller than the first electric power is supplied within a predetermined range from the frequency that is higher than the resonance frequency in a state in which the secondary side of the piezoelectric transformer is open, with regard to the frequency at one point or a plurality of points, and the driving frequency of the piezoelectric transformer for the time when supplying the load with the first electric power is set based on characteristics of the piezoelectric transformer, for example, measurements of impedance or voltage etc. Accordingly, the piezoelectric transformer can be driven with high driving efficiency by corresponding to the characteristic variance arising from the shape or the material characteristics etc. of the piezoelectric transformer.
According to the third driving method and circuit of the piezoelectric transformer, the piezoelectric transformer to be used is classified in advance based on a characteristic parameter, for example, frequency characteristics such as impedance or an electric current etc., and by setting the characteristic parameter relative to the frequency in advance for the classified piezoelectric transformer, piezoelectric transformers with different specifications also can be driven with high driving efficiency.
By incorporating a cold cathode tube emission device that controls the emission of a cold cathode tube by the first, the second or the third driving circuit described above into a liquid crystal panel, and by incorporating this liquid crystal panel into an apparatus such as a mobile phone or a communication terminal etc., the driving frequency is not swept when lighting the cold cathode tube, so that negative effects on the apparatus caused by noise or a cross modulation can be eliminated.